Known, commercially available tracking systems of this type are delivered pre-calibrated, i.e. the exact parameters of the arrangement of their components are known and are taken into account when ascertaining spatial co-ordinates for tracking markers. One general problem with such stereoscopic tracking systems which are pre-calibrated when they are manufactured is that of verifying the stability of the calibration over a longer period of use (“in the field”). Aside from a few very basic functional tests, there is no way of checking the accuracy of the known systems. However, since medical applications of such tracking systems increasingly have accuracy requirements in the sub-millimeter range, verifying the accuracy and/or calibration status is becoming more and more important.
Currently, tracking systems are in most cases checked annually with the aid of a large co-ordinate gauging system which is typically also used for calibrating such systems. However, this cannot be performed in the field, i.e. in situ where the user is; rather, the tracking systems have to be delivered to the manufacturer, where such a co-ordinate gauging system is available. This is associated with a great deal of effort and in particular high costs, and the systems are not available to the user for the corresponding period of time. At the manufacturer's, or wherever the calibration can be performed, the tested device is gauged on the basis of a substantially better (typically, ten times better) reference standard and is possibly recalibrated. However, because the accuracy and stability of the tracking systems are gaining ever-greater importance, because the manufacturer's calibrations mentioned or recalibrations are expensive and incur a downtime, and because non-calibrated systems incur the danger of treatment errors, it is exceedingly important to enable the calibration status of the optical tracking system to be verified in situ where the user is, and with as little effort as possible.